Emus transducer system as well as a method for producing linearly polarised transverse waves with variably predeterminable polarisation direction within a test specimen

ABSTRACT

Described are an EMUS transducer system as well as a method for producing linearly polarised transverse waves with variably predeterminable polarisation direction within a test specimen containing at least ferromagnetic material portions and with a test specimen surface with a magnetisation unit which is provided and set up for being arrangeable on the surface of the test specimen and for producing a magnetic field orientated parallel to the test specimen surface within the test specimen, as well as with at least one HF coil arrangement which is provided and set up for being arrangeable on the test specimen surface and for producing or detecting a HF field occurring overlaid with the magnetic field. 
     The invention stands out in that the magnetisation unit has at least three magnetisation bodies spatially distanced from one another, which are provided and set up to be placeable onto the test specimen surface and in each case to introduce magnetic field lines with a predeterminable magnetic flux into the test specimen, and in that means are provided, using which the magnetic flux of the magnetic field lines to be introduced into the test specimen, which emanate from at least one magnetisation body, can be changed in such a manner, so that the spatial direction of the magnetic field forming parallel to the test specimen surface within the test specimen can be changed in a predeterminable manner.

TECHNICAL FIELD

The invention relates to an EMUS transducer system as well as a methodfor producing linearly polarised transverse waves with variablypredeterminable polarisation direction within a test specimen containingat least ferromagnetic material portions and with a test specimensurface with a magnetisation unit which is provided and set up for beingarrangeable on the surface of the test specimen and for producing amagnetic field orientated parallel to the test specimen surface withinthe test specimen, as well as with at least one HF coil arrangementwhich is provided and set up for being arrangeable on the test specimensurface and for producing or detecting a HF field occurring overlaidwith the magnetic field.

Linearly polarised ultrasound (US) transverse waves are preferably usedfor non-destructive material testing and also for assessing materialstates, such as textures or inner stresses. To this end, it is necessaryto orientate the polarisation direction of the linearly polarised UStransverse waves to be coupled into the material to be investigated ineach case parallel and perpendicularly to the preferential direction ofthe respective texture or inner stress, in order in this manner to beable to detect and correspondingly evaluate propagation differences ofthe US waves characteristic for the anisotropic material in therespective propagation directions. In addition, by suitable orientation,that is to say rotation of the polarisation direction of the US wavesrelatively to the material structure, such as for example workpiecesmade from cast iron, optimum propagation conditions for US waves can becreated, in order to bring about certain evaluable interactions betweenthe US waves and the respective workpiece. It is also possible to detectthe orientation of cracks running perpendicularly to the workpiecesurface into the interior of the material in a spatially resolvedmanner.

PRIOR ART

Linearly polarised US transverse waves are typically produced by meansof piezoelectric or electromagnetic ultrasound transducers.

The use of piezoelectric ultrasound transducers necessitates the use ofa coupling means, using which the ultrasound signals or vibrationsproduced on the part of a piezoelectric ultrasound probe can be coupledinto a workpiece to be investigated or out of the same again. However,the coupling means as well as the coupling gap thickness between theworkpiece surface and the ultrasound probe influence the signal qualityand therefore the validity of the measurement results in adisadvantageous manner. Especially in the case of transverse waves,there is furthermore a requirement for a coupling medium which is ashighly-viscous as possible, as a result of which the test conditions aremade more difficult, particularly in the case of probe movements. Inaddition, the coupling means contaminates the workpiece surface, acircumstance which occurs undesirably in particular in the case ofinvestigations on quality surfaces. Additionally, the use of couplingmeans limits the use of this investigation method on workpieces athigher temperatures.

An alternative to this results due to the use of electromagneticultrasound transducers (EMUS), using which transversely polarisedultrasound waves guided in a mode-pure manner can be excitedparticularly well. In this technology, ultrasound signals are excitedand picked up directly in the layers of a component close to the surfacewithout coupling means, that is to say contactlessly by means ofelectromagnetic interaction. In ferromagnetic materials, the excitationis mainly based on the effect of magnetostriction, by contrast theLorentz force acts as the excitation mechanism in non-magnetic,electrically conductive materials. More information on this can be drawnfrom the following articles: Hirao, M. and Ogi, H. (2003), EMATS forScience and Industry (Kluwer Academic Publishers) and also Igarashi, B.,Alers, G. A., “Excitation of Bulk Shear Waves in Steel byMagnetostrictive Coupling”, IEEE Ultrasonic Symposium Proceedings (1998)893-896.

In FIG. 2, an EMUS transducer arrangement known per se for producingtransversely polarised ultrasound waves within a workpiece 2 consistingof ferromagnetic material is illustrated. The EMUS transducer consistsof a magnetisation unit 1 and also a HF coil system 3. The magnetisationunit 1, which can be constructed in the form of a permanent magnet or anelectromagnet operated with direct current or low-frequency alternatingcurrent, provides two regions 11, 12 which can be placed onto the testsample surface, of which one constitutes the magnetic north pole N andthe other constitutes the magnetic south pole S. In this manner, amagnetic field 13 orientated parallel to the test sample surface isintroduced into the test specimen 2. The HF coil system 3, which canconsist of a single coil or of a plurality of coils, is located betweenthe magnetic north and south poles of the magnetisation unit 1 as closeto the surface as possible on the test specimen surface, in order toproduce a high-frequency electromagnetic field, by means of which themagnetic field 13 orientated parallel to the test specimen surface ismodulated, in the region of the parallel orientated magnetic field 13within the test specimen 2 by corresponding electrical excitation,preferably by means of a powerful HF burst signal. Due tomagnetostrictive effects acting within the ferromagnetic material of thetest specimen 2, oscillating forces arise due to the modulation of thequasi-stationary magnetic field 13 by the high-frequency electromagneticfield, which oscillating forces are used as sources for ultrasoundsignals, particularly in the form of transversely polarised ultrasoundwaves 31 propagating perpendicularly to the test specimen surface. Inthe case of reception, the processes reciprocal to this proceed. Moredetails about this can be drawn from the following article: Niese, F.,Yashan, A., Wilhelms, H., “Wall thickness measurement sensor forpipeline inspection using EMAT technology in combination with pulsededdy current and magnetic flux leakage”, 9th European Conference on NDT,2006, Berlin.

In order to be able to perform material investigations with the aid ofan EMUS probe arrangement of the type indicated at the beginning, it isnecessary to rotate the entire EMUS probe relatively to the testspecimen surface, particularly as the polarisation direction of alinearly polarised transverse wave 31 created with an EMUS probe of thetype of construction illustrated in FIG. 2 is always aligned parallel tothe magnetisation direction, that is to say along the parallel runningmagnetic lines of the magnetic field 13 within the test specimen 2. Arotation at least of the magnetisation unit 1 is in many cases connectedwith additional mechanical outlay and furthermore undesirable,particularly as the measurement constellation relatively to the testobject is subject to a change as a result, which change is error-pronefor an exact investigation of location-dependent material states in testspecimens.

A test apparatus for testing ferromagnetic workpieces by means ofultrasound waves can be drawn from DE 41 01 942, which provides anoverlaying of an alternating magnetic field orientated parallel to theworkpiece surface with a stationary or quasi-stationary magnetic fieldorientated perpendicularly to the workpiece surface for the obliquesonic impingement of vertically polarised ultrasound waves into theworkpiece to be investigated, whereby a HF coil arrangement for couplingeddy currents into the workpiece is attached in the region of themagnetic field orientated perpendicularly to the workpiece surface. Forthe direction-selective sonic impingement, high-frequency transmittingpulses are triggered for controlling the HF coil arrangement either inthe region of the lower or upper half waves of the alternating current.It is neither possible to create ultrasound waves propagating into theworkpiece vertically to the workpiece surface with a test arrangement ofthis type, nor is influence on the spatial orientation of thepolarisation direction provided.

DESCRIPTION OF THE INVENTION

The object is to specify an EMUS transducer system as well as a methodfor producing linearly polarised transverse waves with variablypredeterminable polarisation direction within a test specimen containingat least ferromagnetic material portions in such a manner that amechanical rotation of the test specimen arrangement for purposes of achange of the polarisation direction of the ultrasound waves producedwithin the test specimen should be prevented to the greatest extentpossible. The aim is to be able to perform measurements on testspecimens by means of linearly polarised transverse waves with variablypredeterminable polarisation directions in each case, without moving theEMUS transducer system and in particular the magnetisation unitrelatively to the test specimen.

The solution of the object on which the invention is based is specifiedin claim 1. The subject matter of claim 8 is a method according to thesolution for producing ultrasound waves in the form of linearlypolarised transverse waves with variably predeterminable polarisationdirection. Features which are able to develop the solution idea in anadvantageous manner are the subject matter of the subclaims and also tobe drawn from the further description with reference to the exemplaryembodiments.

The EMUS transducer system according to the solution for producinglinearly polarised transverse waves with variably predeterminablepolarisation direction within a test specimen containing at leastferromagnetic material portions and with a test specimen surface, whichEMUS transducer system has the features known per se of the preamble ofclaim 1, stands out according to the solution in that the magnetisationunit has at least three magnetisation bodies spatially distanced fromone another, which are provided and set up to be placeable onto the testspecimen surface and in each case to introduce magnetic field lines witha predeterminable magnetic flux into the test specimen. Further, meansare provided, using which the magnetic flux of the magnetic field linesto be introduced into the test specimen, which emanate from at least onemagnetisation body, can be changed in such a manner, so that the spatialdirection of the magnetic field forming parallel to the test specimensurface within the test specimen can be changed in a predeterminablemanner.

In order to be able to perform the rotation of the magnetic fieldwithout a mechanical movement of the magnetisation unit, the magneticfield within the test specimen, which forms hitherto between themagnetic poles of only two magnetisation bodies placed on the testspecimen surface, is overlaid in a controllable manner by a furthermagnetic field which is introduced into the test specimen at least by afurther magnetisation body, so that one resulting magnetic field formsfrom the at least two magnetic fields, the magnetic field lines of whichmagnetic field are likewise orientated parallel to the test specimensurface, but with a changed propagation direction, at least within atest specimen region in which the high-frequency electromagnetic fieldwhich can be produced for the part of the HF coil system is able topropagate effectively and enters into interaction with the resultingmagnetic field. For the controllable overlaying of the individualmagnetic fields, means, using which the strength of the magnetic flux ofthe magnetic fields to be introduced into the test specimen in each casecan be varied, are located on at least one, preferably on all of themagnetisation bodies. In this manner it is possible, as the furtherstatements will show, to influence the spatial orientation of themagnetic field lines of the magnetic field responsible for producing thelinearly polarised transverse waves and therefore also the polarisationdirection of the transverse waves themselves in a targeted mannerexclusively by means of suitable variation of the magnetic flux strengthof the individual magnetic fields contributing to the formation of theresulting magnetic field.

The term “magnetising body” is here very generally understood to mean acorporeally constructed magnet which has at least one freely accessiblesurface which at the same time corresponds to the region of a magneticpole. The freely accessible surface is preferably constructed in such amanner that it can be placed onto the test specimen surface over aslarge an area as is possible, that is to say in a contour fittingmanner, in order to enable magnetic field coupling into the testspecimen which is as low in losses or as free of losses as possible.Fundamentally, it is possible to construct the magnetisation bodiesseparately from one another, for example in the form of bar-shapedmagnet bodies.

In a preferred embodiment, the magnetisation bodies are in each caseconnected in pairs to their magnet body regions opposite the freelyaccessible surfaces in each case via a magnetically effective connectingyoke, in order to strengthen the magnetic flux to be introduced via bothmagnetisation bodies into the test specimen.

Two magnetisation body pairs connected in this manner are arrangedrelatively to the test specimen surface in such a manner so that theirmagnetically active connecting yokes cross or cross over in projectiononto the test specimen surface. A possible embodiment variant providesto realise the connecting yokes in one piece in each case, withconstruction of a so-called cross yoke so that all magnetisation bodiesare integrally connected to one another via their respective connectingyokes. Another embodiment variant provides a separate construction oftwo magnetisation body pairs with different overall heights in eachcase, which enable a spatially separate cross over point with respect totheir magnetically active connecting yokes.

For the variation of the magnetic flux of the magnetic fields to beintroduced into the test specimen via the magnetisation bodies in eachcase, means for influencing the strength of the magnetic flux areprovided at least on one, preferably however on all magnetisationbodies. As the further embodiments will show, the means concern theelectrically activatable magnetic coils attached to the magnetisationbodies in the case of electromagnetically constructed magnetisationbodies and in the case of permanent magnetic magnetisation bodies,flux-conducting pieces, which enter into magnetic interaction with themagnetisation bodies and are movably arranged, are provided.

Although the exemplary embodiments explained in the following arelimited to EMUS transducer systems with a magnetisation unit, which isconstructed on the basis of two magnetisation body pairs arranged in acrossed state in each case, it is possible to construct only three oreven more than four magnetisation bodies alone or in a star-shapedmutual yoke connection. All EMUS transducer systems constructed inaccordance with the solution are based on the common solution idea ofgenerating a uniform resulting magnetic field, the magnetic field linesof which are orientated parallel to the test specimen surface and can berotated in a controlled manner about an axis orientated perpendicularlyto the test specimen surface, by controlled overlaying of at least twocontrollable magnetic fields.

A method according to the solution based on this operating principle forproducing ultrasound waves in the form of linearly polarised transversewaves with variably predeterminable polarisation direction consequentlystands out in that at least two magnetic fields orientated parallel tothe test specimen surface are produced within the test specimen, theassignable magnetic field lines of which cut at an angle α≠0°,preferably α=90° and in that a change of the magnetic flux of at leastone of the two magnetic fields is performed in such a manner that auniform magnetic field orientated parallel to the test specimen surfaceresults with a uniformly predeterminable magnetic field lineorientation. The production of the magnetic fields acting in the testspecimen can either be realised with the aid of an electromagnetarrangement or a permanent magnet arrangement. In the case of anelectromagnet arrangement, the change of the magnetic flux of the atleast one magnetic field involved is performed by controlled variationof an electric supply voltage and/or electric current feeding theelectromagnet arrangement. In the case of the production of the magneticfields by means of a permanent magnet arrangement, it is valid forchanging the magnetic flux of at least one of the magnetic fieldsinvolved to perform a spatial variation of at least one permanent magnetor a magnetic flux-conducting piece. Concrete realisation forms are tobe drawn from the exemplary embodiments described in the following. Itis also conceivable to combine both principles for magnetic fieldproduction with one another, in that e.g. a magnetisation body pair iscomposed of two magnetisation body pairs made up of permanent magnetsarranged in a crossed state in each case and the other magnetisationbody pair is constructed in the form of an electromagnet arrangement.

SHORT DESCRIPTION OF THE INVENTION

The invention is described by way of example in the following withoutlimitation of the general inventive idea on the basis of exemplaryembodiments with reference to the drawings. In the figures:

FIG. 1 shows an EMUS transducer system in the form of an electromagnetarrangement in the form of a one-piece cross yoke,

FIG. 2 shows an EMUS transducer (prior art),

FIG. 3 a, b, c show image sequence illustrations for illustrating thespatial position change of the magnetic field orientation by means of anelectromagnet arrangement,

FIG. 4 shows an EMUS transducer system in the form of an electromagnetarrangement with separately constructed magnetisation body pairs incrossed state, and also

FIG. 5 shows an EMUS transducer system in the form of a permanent magnetarrangement.

WAYS OF REALISING THE INVENTION, INDUSTRIAL USABILITY

FIG. 1 shows an EMUS transducer system in perspective oblique view,which has more than four individual magnetisation bodies 1 a, 1 b, 1 cand 1 d, which bear in a flush manner against the test specimen surfaceof a test specimen 2 by means of their ends free at the end face in eachcase. The magnetisation bodies 1 a, 1 b, 1 c, 1 d are realised in alongitudinal, preferably cuboidal or bar-like manner and furthermoreintegrally connected to a cross yoke 1 x at their end regions facingaway from the test specimen 2 in each case. The magnetisation bodies 1a, 1 b, 1 c, 1 d and also the cross yoke 1 x are manufactured from aferromagnetic material and constitute an electromagnet arrangement inconnection with magnetisation coils 4 which are in each case arrangedaround the individual magnetisation bodies 1 a-1 d. Taking account of avoltage unit supplying the individual magnetisation coils 4 withelectrical energy has been dispensed with for reasons of better clarity.

In projection onto the test specimen surface centrally below the crossyoke arrangement 1 x, a HF coil system 3 is located on the test specimensurface, the high-frequency field of which is able to modulate themagnetic field prevailing within the test specimen 2, as a result ofwhich linearly polarised ultrasound transverse waves are ultimatelyproduced within the test specimen 2.

To illustrate the change of the spatial orientation of the magneticfield prevailing within the test specimen 2, reference may be made tothe image sequence illustration in the FIGS. 3 a-3 c, in which the fourmagnetisation bodies 1 a-1 d are illustrated in each case in projectiononto the test specimen surface with the magnetisation coils 4 whichsurround them in each case. In the image sequence illustration accordingto FIG. 3 a, it may be assumed that the magnetisation coils of themagnetisation bodies 1 a, 1 c are switched in a currentless manner, sothat no magnetic field prevails between these two magnetisation bodies 1a, 1 c. The magnetisation coils 4 are activated with respect to themagnetisation bodies 1 b, 1 d so that the magnetisation body 1 b forms amagnetic south pole on the test specimen surface and also themagnetisation body 1 d forms a magnetic north pole. Thus, there resultsa magnetic field 31 orientated parallel to the test specimen surface,the magnetic field lines of which are orientated from north to south,that is to say from top to bottom in the case illustrated according toFIG. 3 a.

If, in addition to the activated magnetic coils on the magnetisationbodies 1 b, 1 d, those of the magnetisation bodies 1 a, 1 c are alsoactivated, as is illustrated in FIG. 3 b, then two magnetic fieldsoverlay to form a resulting magnetic field 13 in the central region ofall four magnetisation bodies 1 a-1 d in the manner indicated in FIG. 3b. In this case, a magnetic field 13 spatially rotated through 45°compared to the image sequence according to FIG. 3 a is set.

If, as in the case of the sequence according to FIG. 3 c, themagnetisation coils of the magnetisation bodies 1 b and 1 d are switchedcurrentlessly, then the magnetic field between the magnetisation bodiesla and 1 c prevails exclusively, that is to say the magnetic field 13orientated parallel to the test specimen surface then points from leftto right. When the sequence of images of FIGS. 3 a to 3 c are viewedtogether, the orientation of the magnetic field 13 has therefore beenrotated through exactly 90°, without spatially moving the magnetisationbodies 1 a, 1 b, 1 c, 1 d in the process.

FIG. 4 shows a further embodiment which like the exemplary embodimentillustrated in FIG. 1 has four magnetisation bodies 1 a to 1 d, whichare not connected to one another via an integral cross yoke however, butrather in each case in pairs via magnetically active connecting yokes 1acv, 1 bsv. To realise the crossed state, like those in FIG. 1, themagnetisation bodies 1 b and 1 d are constructed in a longer mannercompared to the magnetisation bodies 1 a, 1 c, so that the individualmagnetically active connecting yokes 1 acv, 1 bdv cross over in aregular manner. The mechanism of action is identical to that which hasbeen explained with reference to the image sequence illustrationaccording to FIGS. 3 a to 3 c.

In FIG. 5, an embodiment for an EMUS transducer system with amagnetisation unit is shown, which consists exclusively of permanentmagnets 1 a′, 1 b′, 1 c′ and 1 d′. The magnetic poling of the permanentmagnets 1 a′ to 1 d′ constructed as bar magnets result on the basis ofthe designations for N=north pole and S=south pole contained in FIG. 5.A first magnetic field orientated parallel to the test specimen surfacewithin the test specimen 2 is generated by the permanent magnets 1 a′and 1 c′ in the x direction and a second magnetic field orientatedparallel to the test specimen surface is generated by the permanentmagnets 1 b′ and 1 d′ in the y direction. The x and y directions areorientated orthogonally to one another.

In addition, the permanent magnet pairs 1 a′/1 c′ and also 1 b′/1 d′ arein each case connected to one another via separately constructedmagnetically active connecting yokes 1 acv′, 1 bdv′.

In order to be able to pivot the direction of the magnetic field at thelocation of the HF coil system 3 within the test specimen 2 in acontrolled manner, it is valid to vary the magnetic flux through therespectively separately constructed connecting yokes 1 acv′, 1 bdv′.This is achieved in that each individual part of the connecting yokes 1acv′, 1 bdv′ is spatially movably mounted via an actuator which is notillustrated further. If both parts of the respective connecting yoke arein direct contact, as is the case in FIG. 5 for the permanent magnets 1b′ and 1 d′ illustrated, then the magnetic flux in this connecting yoke1 bdv′ considered is maximal. Consequently, the magnetic flux which isintroduced by means of the permanent magnets 1 b′ and 1 d′ into the testspecimen 2 is also maximal. If, however, there is an air gap between thetwo parts of the connecting yoke, as is the case in FIG. 5 for thepermanent magnets 1 a′ and 1 c′, then the magnetic flux is reduced as afunction of the gap width. In the case of FIG. 5, the test specimen 2 isprimarily magnetised by the permanent magnets 1 b/1 d short-circuited bymeans of the connecting yoke 1 bdv′. This connecting yoke 1 bdv′therefore also specifies the direction of the magnetic field in the testbody at the location of the HF coil system. If, by contrast, the air gapis closed in all of the connecting yokes, then the magnetic fields ofboth yokes or both permanent magnet pairs 1 a′/1 c′ and 1 b′/1 d′ areoverlaid, so that a magnetic field direction of 45° is set, comparablyto the image sequence illustration in FIG. 3 b.

With a suitable actuator, with which it is possible to vary the positionof the parts of the connecting yokes 1 acv′, 1 bdv′ in the mannerdescribed previously, direction and field strength of the resultingmagnetic field within the test specimen 2 at the location of the HF coilsystem 3 can be changed with mechanical means exclusively.

As already mentioned previously, it is fundamentally possible to choosethe number of magnetisation bodies involved arbitrarily for producing aresulting magnetic field within the test specimen 2, that is to say atleast n=3 magnetisation bodies, preferably however an even number n.

A series of advantages is connected with the EMUS transducer systemaccording to the solution. So, the EMUS transducer system does not haveto be mechanically rotated with respect to the test specimen in order tochange the polarisation direction of the transverse wave. The positionof the EMUS transducer system can be maintained exactly during therotation of the polarisation direction, as the transducer itself is notmoved.

The transducer principle can be used for automatic test systems withoutcoupling means, so that it is also available for applications at hightemperatures.

The main directions of anisotropic test specimens do not need to beknown before the test. The EMUS transducer system can be placed andpositioned onto an otherwise unknown test body. Nonetheless, it ispossible to discover the main directions by stationary rotation of thepolarisation direction. Fundamentally, any polarisation directions ofthe linearly polarised transverse waves can be set.

Further, it is possible by measuring the speed of sound as a function ofpolarisation direction to detect textures and/or inner stresses inaccordance with thickness and direction. For fault checking, thepossible direction of a crack can be detected fully automatically by therotation, in accordance with the solution, of the polarisationdirection. Further, e.g. in the case of strongly anisotropic materials,the optimum polarisation direction for the test task can be discoveredwithout an elaborate mechanical rotation of the entire probe.

REFERENCE LIST

1 Magnetisation unit

1 a,b,c,d Magnetisation body

1 a′,b′,c′,d′ Permanent magnets

1 x Cross yoke

1 acv, 1 bdv Magnetically active connecting yoke

1 acv′, 1 bdv′ Separated connecting yoke

13 Magnetic field

2 Probe

3 HF coil system

4 Magnetisation coils

1. EMUS transducer system for producing linearly polarized transverse waves with variably predeterminable polarization direction within a test specimen containing at least ferromagnetic material portions, with a test specimen surface, with a magnetization unit which is provided and set up for being arrangeable on the surface of the test specimen and for producing a magnetic field orientated parallel to the test specimen surface within the test specimen, as well as with at least one HF coil arrangement which is provided and set up for being arrangeable on the test specimen surface and for producing or detecting a HF field occurring overlaid with the magnetic field orientated parallel to the test specimen surface within the test specimen, characterized in that the magnetization unit has at least three, preferably four, magnetization bodies spatially distanced from one another, which are provided and set up to be placeable onto the test specimen surface and in each case to introduce magnetic field lines with a predeterminable magnetic flux into the test specimen, and in that means are provided, using which the magnetic flux of the magnetic field lines to be introduced into the test specimen, which emanate from at least one magnetization body, can be changed in such a manner, so that the spatial direction of the magnetic field forming parallel to the test specimen surface within the test specimen can be changed in a predeterminable manner.
 2. EMUS transducer system according to claim 1, characterized in that the magnetization bodies in each case have a freely accessible surface at the end face, which is constructed for placing onto the test specimen surface, as well as a region opposite the freely accessible surface which is or can be connected via at least one magnetically active connecting yoke to at least one other magnetization body.
 3. EMUS transducer system according to claim 1, characterized in that the magnetization unit is constructed in the manner of an electromagnet arrangement, in that the means for changing the magnetic flux of the magnetic field lines to be introduced into the test specimen are constructed in the form of electric coils, of which at least one electric coil is attached to one magnetization body in each case, and in that the electric coils are connected to a controllable direct current or alternating current source.
 4. EMUS transducer system according to claim 1, characterized in that the magnetization bodies are constructed in the form of a permanent magnet in each case, and in that the means for changing the magnetic flux of the magnetic field lines to be introduced into the test specimen are constructed in the form of at least one magnetic flux conducting piece, the spatial location of which can be changed relatively to the at least one magnetization body.
 5. EMUS transducer system according to claim 4, characterized in that the magnetic flux conducting pieces of at least two magnetization bodies are parts of a magnetically active connecting yoke, and in that a controllable kinematic unit is provided, by means of which the spatial position of at least one magnetic flux conducting piece can be changed.
 6. EMUS transducer system according to claim 1, characterized in that the magnetization unit has an electromagnet arrangement and a permanent magnet arrangement, in that the means for changing the magnetic flux of the magnetic field lines to be introduced into the test specimen are constructed in the form of electric coils in the case of the electromagnet arrangement, of which at least one electric coil is attached to one magnetization body in each case, and in that the electric coils are connected to a controllable direct current or alternating current source, and in that the means for changing the magnetic flux of the magnetic field lines to be introduced into the test specimen are constructed in the form of at least one magnetic flux conducting piece in the case of the permanent magnet arrangement, the spatial location of which can be changed relatively to the at least one magnetization body.
 7. EMUS transducer system according to any one of claim 1, characterized in that at least two magnetization bodies are or can be connected via a magnetically active connecting yoke and form a magnetization body pair, in that at least two magnetization body pairs are constructed and arranged in such a manner that the magnetically active connecting yokes of the at least two magnetization body pairs cross or cross over in projection onto the test specimen surface.
 8. Method for producing ultrasound waves in the form of linearly polarized transverse waves with variably predeterminable polarization direction within a test specimen containing at least ferromagnetic material portions using electromagnetic ultrasound creation in which a magnetic field orientated parallel to a surface of the test specimen is produced within the test specimen and a high-frequency electromagnetic field is produced with the aid of a HF coil arrangement, by means of which the magnetic field parallel to the test specimen surface is modulated, characterized in that at least two magnetic fields orientated parallel to the test specimen surface are produced within the test specimen, the assignable magnetic field lines of which cut at an angle a which does not equal 0°, preferably 90°, and in that a change of the magnetic flux of at least one of the two magnetic fields is performed in such a manner that a uniform magnetic field orientated parallel to the test specimen surface results with a uniformly predeterminable magnetic field line orientation.
 9. Method according to claim 8, characterized in that the at least two magnetic fields orientated parallel to the test specimen surface are produced within the test specimen by means of an electromagnet arrangement, and in that the change of the magnetic flux of the at least one magnetic field is performed by adaptation of the electric supply voltage of the electromagnet arrangement producing the at least one magnetic field.
 10. Method according to claim 8, characterized in that the at least two magnetic fields orientated parallel to the test specimen surface are produced within the test specimen by means of a permanent magnet arrangement, and in that the change of the magnetic flux of the at least one magnetic field is performed by spatial variation of at least one permanent magnet or a magnetic flux conducting piece.
 11. Method according to claim 8, in that the at least one magnetic field orientated parallel to the test specimen surface is produced within the test specimen by means of a permanent magnet arrangement and at least one further magnetic field orientated parallel to the test specimen surface is produced within the test specimen by means of an electromagnet arrangement, and in that the change of the magnetic flux of the at least one magnetic field produced by the permanent magnet arrangement is performed by spatial variation of at least one permanent magnet or a magnetic flux conducting piece and/or in that the change of the magnetic flux of the at least one magnetic field produced by the electromagnet arrangement is performed by adaptation of the electric supply voltage of the electromagnet arrangement producing the at least one magnetic field. 